1. Field of the Invention
The present invention relates to a control system in an AC servomotor having a permanent magnet type rotor, and more particularly, to a control system enabling a high-torque operation at a high speed by controlling a DC voltage at a connecting point between a converter and an inverter.
2. Description of the Related Art
Servomotors are widely used in various kinds of numerical control (NC) machine tools, working robots, or the like, to position the tables of the NC machine tools, and the attitudes of the working robots.
Currently, an AC servomotor with a permanent magnet type rotor is mainly used, because the maintenance thereof is easy due to the use of electronic switches such as power transistors, and the absence of mechanical commutators and brushes.
To operate an AC servomotor at a required speed and a required torque, it is well known to control an ON-OFF period of transistor switches with a pulse wide modulation (hereinafter referred to as PWM) (see Japanese Unexamined Utility Model Application (Kokai) 57-135892).
In this control system, AC power is rectified by the converter having diodes which consist of a three-phase bridge circuit. The rectified DC power is again inverted by the inverter with transistor switches which also consist of a three-phase bridge circuit, to supply an AC power to the AC servomotor, and the transistor switches are controlled in accordance with the switching pattern determined by the PWM, based on a target speed and torque.
In this control system, however, the converter generally consists of diodes, and DC voltage at the output terminal of the converter is unconditionally determined by the inter-phase AC voltage at the input of the inverter. Therefore, the AC output voltage at the output of the inverter, which is applied to the AC servomotor, cannot be controlled beyond the voltage determined by the inter-phase AC voltage at the input of the inverter. Accordingly, the output torque is lowered when the AC servomotor is operated over the basic speed (hereinafter referred to as .omega..sub.b)
FIG. 5 shows a characteristic of a typical AC servomotor with a permanent magnet rotor, wherein the dotted line shows the characteristic when the AC servomotor is operated by a conventional control method. This characteristic shows that the AC servomotor is operated at a constant torque under .omega..sub.b, but does not maintain a constant torque over .omega..sub.b.